1. Field of the Invention
The present invention relates to a signal processing apparatus for interpolating detection signals of encoders including a rotary encoder for detecting the rotational position of a motor of a machine tool such as an NC device, a linear scale for detecting the linear movement position of a table, etc.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a position and angle detecting method for a conventional encoder. Original signals of a sine wave Sa and a cosine wave Sb that are delivered from a sensing element of the encoder are amplified by means of analog amplifiers 1a and 1b so that they may match the respective inputs of analog-digital converters 2a and 2b in the next stage. An analog amplifier 7 corrects the offset, amplitude difference, and phase difference of the amplified original signals Sa and Sb, thereby approximating the sine and cosine waves to ideal ones. The analog signals are converted digital signals by means of the analog-digital converters 2a and 2b, and positions in each original-signal period (each sine-wave period) are calculated by means of a digital interpolator 3. A position data generator 6 originates and outputs position data in accordance with calculated position data for one period and data in a counter (not shown in this block diagram) that counts the periods of the original signals.
Described in Japanese Patent Application Laid-Open No. 10-311741, moreover, is a method in which position data adjusted in offset and amplitude are obtained in a manner such that the original signals from the sensing element of the encoder are converted into digital signals, and the offset value and amplitude ratio are obtained in accordance with the converted digital signals.
The above-described method that is used in the conventional encoder and employs the analog circuits for the adjustment of offset voltage, amplitude difference, and phase difference requires large-sized circuits, so that it is subject to the problems of a wider packaging area and higher cost. Further, the method has problems that it cannot remove waveform distortion (harmonic component) and that it cannot cope with waveform changes attributable to temperature fluctuations during operation and waveform changes with time. The aforesaid method described in Japanese Paten Application Laid-Open No. 10-311741, moreover, serves for the adjustment of offset and amplitude, and involves no consideration of the adjustment of phase difference.